Pressure fluid cylinders of the type concerned in US patent 4 164 893 and comprising a cylinder having a longitudinal slot through which a carrier means arranged in a piston, which is movable in the cylinder, extends, the slot being sealed by means of a sealing band and the piston being provided with a sealing cuff at each end, are beset with the shortcoming that the thickness of the carrier means is limited by the width or breadth of the slot. It is desired to make the carrier means broader in order that it might absorb greater lateral loads. However, it is not possible with the present embodiment of the cylinder to widen the slot without being obliged to make the sealing band broader. A broader sealing band brings a greater internal ovality with a greater difficulty in managing the axial seal of the sealing cuffs of the piston and the ends of the cylinder and a risk of deformation or outward bending at overpressure in the cylinder.